The 18mm plugs were too long ago even for me to have had a lot to do with them: even in the 1930s, 14mm was more usual. The only engine with an 18mm plug I can clearly remember owning, was a 2 stroke Villiers on a 1949 12" ATCO.
Don't bet those old Lodge plugs your friend has will be dismantlable. I think Lodge disappeared some years after the industry switched to non-dismantlable.
